Well, with all due respect, and not wishing to sound too snarky, a factual answer to your OP would still be of no use to you.
Let’s just say that the Teeming Millions had come up with a factual answer by crunching the numbers on every single movie ever made (an impossible task I know, but bear with me) and come up with an answer of, say, 1hr 47min and 33.659 seconds.
How would you use that information? The fact is, your friend is watching a specific movie, not “the average” movie.
I would recommend that, in future, you try one of two alternatives:
[li]Ask her what movie she’s going to watch, then look up the length on imdb.com, and add a few minutes for bathroom breaks.[/li][li]Ask her to call you back when the movie is over.[/li][/ul]